Patent GB-706381-A (GB)
Title: Improvements in or relating to electric pulse code modulation systems of communication
Inventor: TREADWELL CYRIL GORDON
IPC: H04J3/10
Filed: 1952-06-24
Abstract: 706,381. Multiplex pulse code siganlling. STANDARD TELEPHONES & CABLES, Ltd. June 24, 1952, No. 15917/52. Class 40 (5). [Also in Group XXXVIII] In a pulse code communication system, signal samples, quantized on an amplitude scale having an equal number of positive and negative steps, are transmitted as a group of one or more digit pulses according to a code of two or more elements, the digit pulses having an amplitude level selected from a series of equal positive and negative values. The pulses may be transmitted over a line or by frequency modulation of a radio carrier, for example. In a m-element code, the pulses having one of n positive and n negative amplitude levels the total number of sample amplitudes which can be used is (2n+1)<SP>m</SP>, including zero. In one of the multiplex systems described the two-element code used comprises positive or negative digit pulses which have four possible amplitudes 3E/2, 5E/2, 7E/2, 9E/2, E being a fixed voltage, and the sample amplitude range is from -40 to +40 steps. The code for the steps 0 to -40 is shown in Fig. 1, curve A representing this range of sample pulse voltages. The pairs of pulses B above, separated by short vertical strokes, represent these voltages on a scale with radix nine. Thus each pair of pulses represents the same voltage 9a+b, where a and b are the digit values which can each have eight integral values -4 ... +4 or zero. As shown the first digit pulse of each pair, when present, is indicated by an arrowhead and timing pulses, shown dotted and of amplitude E/2, are inserted where zero pulse is called for. In the synchronizing channel occur a pair of pulses of amplitude 11E/2. The code for the steps 0 to +40 of the sample amplitude range is obtained by changing the polarity of all the pulses constituting the steps 0 to -40. The system comprises 119 communication and one synchronizing channels, the 120 channel periods each being of one microsecond duration so that the channel recurrence frequency is 8<SP> 1</SP>/ 3  kc/s. In each channel period the two digit or timing pulses are spaced 0.5 microsecond apart and have a duration of 0.25 microsecond. The pulse recurrence frequency of the transmitted train is thus 2 Mc/s. Transmitter.-The timing of the transmitter circuits is controlled by the master generator 1, Fig. 2, the output of which, at 2 Mc/s, is a square wave the alternately positive and negative portions of which are of 0.25 microsecond duration. This timing wave No. 1 is fed to the timing and synchronizing pulse generator 38 over lead 37, and to a frequency divider 2 which produces a similar square wave of double the period, timing wave No. 2, for controlling the multiplex distributer. This wave is also applied to a shaper 3 which produces a train of 0.25 microsecond pulses with a recurrence period of 1 microsecond, timing wave No. 3, which is fed to the coding circuits over leads 20, 30. Each channel signal is applied to the terminal 10 of a channel unit 7 (detailed in Fig. 4) connected to a delay line distributer which may be of the type decribed in Specifications 587,939 and 635,472, so that the channel unit outputs are fed in succession through output leads 17, 18, upon which appear pulses circuits. The channel unit output consists of two simultaneous negative pulses A, B on separate conductors and having amplitudes a when the input signal is zero, and (a+x) and (a-x) respectively for other inputs x. The A and B pulses are applied over leads 11, 12 to a mean level restorer circuit 14 (detailed in Fig. 5) which has a common logarithmic amplifier 13 to the coding A1, B1, respectively of opposite sign, of amplitude x and duration 0.25 microsecond determined by timing wave No. 3 on lead 20. When x is negative, A1 is negative and B1 positive, while when x is positive A1 is positive and B1 negative. The .negative pulse of amplitude x is applied over lead 21 to the coder 22 (detailed in Fig. 6), which generates the pair of code digit pulses the polarity of which is set correctly by the polarity restorer circuit 23 (detailed in Fig. 10). The latter is controlled over leads 27, 28 by the polarity control circuit 19 (detailed in Fig. 9) fed with the A1 and B1 pulses from circuit 14. The first code digit pulse is fed to the F.M. transmitter through the mixer circuit 24 (included in Fig. 10) and also with both polarities, as pulses A2, B2, over leads 31, 32, to the vernier digit amplifier 29 (detailed in Fig. 11). This circuit prepares a voltage to be coded in circuit 22, after a delay of 0.5 microsecond, to provide the second code digit pulse. It is supplied also with A1, B1 pulses over leads 33, 34 and stabilized voltages over leads 43, 257 and provides polarity control pulses A3, B3, over leads 35, 36, to circuit 19. The amplifier 29, rendered active by a pulse of timing wave No. 3, compares the positive sample pulse A1 or Bl with a positive comparison pulse derived from A2 or B2 and of amplitude equal to that which the first digit represents. The amplitude difference is multiplied by nine and appears as the A3, B3 pulses the negative one of which is applied with the 0.5 microsecond delay to the input lead 21 of the coder 22. After coding, and polarity restoration in circuit 23, the second digit pulse thus produced is fed to the mixer 24. The timing and synchronizing pulses are generated in circuit 38 (detailed in Fig. 12) and are fed to the mixer over lead 39. Stabilized potentials are provided by circuit 42 for the coder 22 and vernier digit amplifier 29. Channel unit, Fig. 4.-The channel signal at terminal 10 is applied in push-pull to valves 64, 65, normally blocked by the cathode potential divider 69, 70. These valves provide the simultaneous negative A and B pulses, through rectifiers 75, 76 respectively, when a negative unblocking pulse from valve 77 is applied to their cathodes. Valve 77 is unblocked to provide this pulse when a positive one microsecond pulse, from the distributer, on lead 8 and applied to the grid, coincides with a positive 0.5 microsecond pulse, from timing wave No. 2, on lead 9 applied to rectifier 83, which is thereby blocked to remove the short circuit in the grid-cathode circuit. Mean level restorer, Fig. 5.-The A and B pulses are applied respectively to the grids of valves 91, 92 which are cut off at their suppressor grids and provided with negative feedback by their cathode resistances 98, 99. 0.25 microsecond pulses from timing wave No. 3 over lead 20 release the suppressor grids and pass the A1, B1 pulse from the push-pull output transformer 95 over leads 33, 34 to the vernier digit amplifier, Fig. 11, and over leads 17, 18 to the polarity control circuit, Fig. 9. The coder, Fig. 6, is fed over lead 21 with the negative A1 or B1 pulse selected by one of the oppositely poled rectifiers 96, 97. Coder, Fig. 6.-The coder comprises four similar coding stages 100 ... 103, detailed in Fig. 7 (not shown), Input pulses on lead 21 are fed, through the cathode follower valve 104, to the input terminals 106 of these stages through condensers 107 .. 110 and rectifiers 111 ... 114. The latter are all blocked by potentials of 5e, 14e, 23e and 32e, respectively, derived from terminals 115 ... 118 of the stabilized voltage source 42, detailed in Fig. 8 (not shown), 40e being the maximum amplitude of the pulse at the cathode of valve 104. When the amplitude of a given pulse exceeds the blocking potential of one or more of these rectifiers, the corresponding coding stages are triggered simultaneously to generate 0.5 microsecond pulses at the output terminals 119, 120. The latter four pairs of terminals are connected in series between earth and lead 121 feeding the polarization restorer circuit, Fig. 10. They are shunted by rectifiers 122, 125 respectively so that untriggered stages are bye-passed. Stages 101, 102, 103 generate pulses of amplitude E, while stage 100 generates pulses of amplitude 3E/2, so that the summed pulses have an amplitude 3E/2, 5E/2, 7E/2 or 9E/2 as required by the coding system. The pulse amplitude are determined stabilized voltages derived from a gas-discharge tube 126 and resistances 127, 128, the limiter terminals 129 of the coder stages being connected to appropriate points on these elements through rectifiers 130 ... 133 respectively. Whenever a code pulse i produced a negative pulse is applied, from ter minal 129 of stage 100, to lead 146 for con trolling the polarization control circuit. Polarization control circuit, Fig. 9.-The puls from the coder over lead 146, cuts off the valve 176 to remove a blocking bias across its cathode resistances 185, 186 which are common to the cathodes of the right-hand valves of two multi vibrator pairs 172, 173 and 174, 175. These multivibrators can now be triggered either by a negative A1 pulse on lead 17 connected to valve 172 or a negative B1 pulse on lead 18 connected to valve 174. When the signal sample is negative the A1 pulse is negative and multivibrator 172, 173 is triggered to supply a 0.5 microsecond pulse to output lead 27 and when the sample is positive a similar pulse is supplied from multivibrator 174, 175 to output lead 28. The second digit of each pair of code digits has its polarity controlled by A3, B3 pulses applied respectively to the two multivibrators over leads 35, 36 from the vernier digit amplifier, Fig. 11 Polarization restorer circuit and mixer, Fig 10.-The digit pulses from the coder, on lead 121, are applied in parallel to the control grids of pentodes 198, 199 having a push-pull output transformer 210. The valves are normally cut off by their suppressor grids, which are connected to earth through separate opposed secondaries of a transformer 203 the primary of which is connected to the leads 27, 28 carrying the control pulses from the polarization control circuit, Fig. 9. If a negative pulse appears on lead 27, valve 198 is unblocked and a negative amplified digit pulse is